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Objectives: We herein assessed the analytical performances of the antigen-rapid diagnostic test (Ag-RDT) SIENNA™ COVID-19 Antigen Rapid Test Cassette (Nasopharyngeal Swab) (Salofa Oy, Salo, Finland), targeting the SARS-CoV-2 N nucleocapsid protein, for the diagnosis of COVID-19 in hospitalized patients with suspected SARS-CoV-2 infection, by reference to real-time RT-PCR (rRT-PCR). Methods: Nasopharyngeal swabs were collected from patients with COVID-19-like illness during the second epidemic wave in Paris, France, among which 100 and 50 were positive and negative for SARS-CoV-2 RNA, respectively. Results: Overall, the Ag-RDT showed high sensitivity, specificity, positive and negative predictive values of 90.0%, 100.0%, 100.0% and 98.1%, respectively, as well as high or almost perfect agreement (93.3%), reliability assessed by Cohen’s κ coefficient (0.86), and accuracy assessed by Youden’s J index (90%) to detect SARS-CoV-2. The analytical performances of the Ag-RDT remained high in the event of significant viral excretion (i.e., N gene Ct values ≤33 by reference rtRT-PCR), while the sensitivity of the Ag-RDT dropped to 69.6% with low or very low viral shedding (Ct > 33). Conclusions: The SIENNA™ Ag-RDT presents excellent analytical performances for viral loads ≤33 Ct, classically corresponding to situations of symptomatic COVID-19 and/or proven contagiousness.